Field of the Invention
This invention relates to applying coatings to thin foils. More particularly, this invention relates to applying boron carbide coatings to thin metallic foils. Even more particularly, this invention relates to a continuous process for applying a boron coating to metallic foils which are utilized to manufacture boron-coated straw neutron detectors.
Description of the Related Art
The application of neutron detection technology to the fields of national security, oil/gas exploration, nuclear safeguards, neutron science instrumentation and other areas is greatly expanding. Unfortunately, the neutron detection systems of choice which utilizes pressurized tubes of 3He have several limitations. While these systems can provide the needed spatial resolution and gamma ray discrimination, this technology cannot achieve high rate operation because of slow drift of positive ions. Furthermore, large detection areas are costly, because of the complexity of the pressure vessels required, and parallax errors limit the time-of-flight accuracy of the instrument. Perhaps the most problematic issue for the future of 3He detectors is a severe shortage of 3He. Existing stockpiles of 3 He will soon be depleted and only limited amounts are naturally available or available from decay of U.S. and Russian tritium supplies. Future instrument expansions will likely not afford the escalating cost of the dwindling 3He supply.
Recognizing the problems with 3He detector systems early on, Dr. Jeffery L. Lacy developed a new technology for replacing the 3He detectors. The technology was the boron-coated straw detector. The boron-coated straw (BCS) detector was based on arrays of thin walled boron-coated copper tubes. The elemental component of this detector was a long tube (“straw”), generally about 1 to 4 mm in diameter, coated on the inside with a thin layer of 10B-enriched boron carbide (10B4C).
Thermal neutrons captured in 10B are converted into secondary particles, through the 10B(n,α) reaction:10B+n→7Li+α  (1)The 7Li and α particles are emitted isotropically in opposite directions with kinetic energies of 1.47 MeV and 0.84 MeV, respectively (dictated by the conservation of energy and momentum). For a boron carbide layer that is only about 1 μm thick, one of the two charged particles will escape the wall 78% of the time, and ionize the gas contained within the straw.
Each BCS detector was operated as a proportional counter, with its wall acting as the cathode, and a thin wire tensioned through its center serving as the anode electrode, operated at a high positive potential. Primary electrons liberated in the gas drift to the anode, and in the high electric field close to the anode, avalanche multiplication occurs, delivering a very much amplified charge on the anode wire. Standard charge-sensitive preamplifier and shaping circuitry were used to produce a low noise pulse for each neutron event. Gamma interactions in the wall produced near minimum ionizing electrons that deposit a small fraction of the energy of the heavily ionizing alpha and Li products. Gamma signals were effectively discriminated with a simple pulse height threshold.
The boron-coated straw detector technology was first patented by Dr. Lacy in U.S. Pat. No. 7,002,159 entitled “Boron-Coated Straw Neutron Detector” based upon a Nov. 13, 2002, filing. As the thought leader of this technology area, Dr. Lacy continued his research and development to improve the boron coated straw detectors and to find new uses. Examples of Dr. Lacy's continued progress in this technology area are found in his other issued patents and pending patent applications which include: U.S. Pat. No. 8,330,116 entitled “Long Range Neutron-Gamma Point Source Detection and Imaging Using Rotating Detector”; U.S. patent application Ser. No. 12/792,521 filed Jun. 2, 2010, entitled “Optimized Detection of Fission Neutrons Using Boron-Coated Straw Detectors Distributed in Moderator Material” (allowed and issue fee paid); U.S. patent application Ser. No. 13/106,785 filed May 12, 2011, entitled “Sealed Boron-Coated Straw Detectors”, U.S. patent application Ser. No. 13/106,818 filed May 12, 2011, entitled “Neutron Detectors for Active Interrogation”; and U.S. patent application Ser. No. 13/683,404 filed Nov. 21, 2012, entitled “Boron Coated Straw Detectors with Shaped Straws.” These patent and pending applications mentioned in this paragraph are hereby incorporated by reference in their entirety for all purposes, including but not limited to those portions describing the structure and technical details of the boron-coated straw detectors as background and for use as specific embodiments of the present invention, and those portions describing other aspects of manufacturing and testing of boron-coated straws that may relate to the present invention.
Dr. Lacy also widely published articles on boron-coated straw detection capabilities, fabrication, and development of prototypes for various applications including:    J. L. Lacy, et al, “Novel neutron detector for high rate imaging applications”, IEEE Nuclear Science Symposium Conference Record, 2002, vol. 1, pp. 392-396;    J. L. Lacy, et al, “Straw detector for high rate, high resolution neutron imaging”, in IEEE Nuclear Science Symposium Conference Record, 2005, vol. 2, pp. 623-627;    J. L. Lacy, et al, “High sensitivity portable neutron detector for fissile materials detection”, IEEE Nuclear Science Symposium Conference Record, 2005, vol. 2, pp. 1009-1013;    J. L. Lacy, et al, “Performance of 1 Meter Straw Detector for High Rate Neutron Imaging”, IEEE Nuclear Science Symposium Conference Record, 2006, vol. 1, pp. 20-26;    J. L. Lacy, et al, “Long range neutron-gamma point source detection and imaging using unique rotating detector”, IEEE Nuclear Science Symposium Conference Record, 2007, vol. 1, pp. 185-191,    J. L. Lacy, et al, “Fabrication and materials for a long range neutron-gamma monitor using straw detectors”, IEEE Nuclear Science Symposium Conference Record, 2008, pp. 686-691;    J. L. Lacy, et al, “One meter square high rate neutron imaging panel based on boron straws”, IEEE Nuclear Science Symposium Conference Record, 2009, pp. 1117-1121;    J. L. Lacy, et al, “Boron coated straw detectors as a replacement for 3He”, IEEE Nuclear Science Symposium Conference Record, 2009, pp. 119-125;    J. L. Lacy, et al, “One meter square high rate neutron imaging panel based on boron straws”, IEEE 2009 Nuclear Science Symposium Conference Record, 2009, pp. 1117-1121;    J. L. Lacy, et al, “Initial performance of large area neutron imager based on boron coated straws”, IEEE 2010 Nuclear Science Symposium Conference Record, 2010, pp. 1786-1799;    J. L. Lacy, et al, “Initial performance of sealed straw modules for large area neutron science detectors”, IEEE 2011Nuclear Science Symposium Conference Record, 2011, pp. 431-435;    J. L. Lacy, et al, “Straw-Based Portal Monitor 3He Replacement Detector with Expanded Capability”, IEEE 2011 Nuclear Science Symposium Conference Record, 2011, pp. 431-435;    J. L. Lacy, et al, “Performance of a Straw-Based Portable Neutron Coincidence/Multiplicity Counter”, IEEE 2011 Nuclear Science Symposium Conference Record, 2011, pp. 529-532;    J. L. Lacy, et al, “Replacement of 3He in Constrained-Volume Homeland Security Detectors”, IEEE 2011 Nuclear Science Symposium Conference Record, 2011, pp. 324-325;    J. L. Lacy, et al, “Initial performance of sealed straw modules for large area neutron science detectors”, IEEE 2011 Nuclear Science Symposium Conference Record, 2011, pp. 431-435;    J. L. Lacy, et al, “Boron-coated straws as a replacement for 3He-based neutron detectors”, Nuclear Instruments and Methods in Physics Research, Vol. 652, 2011, pp. 359-363;    J. L. Lacy, et al, “Design and Performance of High-Efficiency Counters Based on Boron-Lined Straw Detectors”, Institute of Nuclear Materials Management Annual Proceedings, 2012;    J. L. Lacy, et al, “Boron-coated straw detectors of backpack monitors”, IEEE Transactions on Nuclear Science, Vol. 60, No. 2, 2013, pp. 1111-1117.    J. L. Lacy, et al, “The Evolution of Neutron Straw Detector Applications in Homeland Security”, IEEE Transactions on Nuclear Science, Vol. 60, No. 2, 2013, pp. 1140-1146.Each of these publications is hereby incorporated by reference into this application in their entirety for all purposes.
The key sensing element in boron-coated neutron detectors has been the very thin (nominally about 1 μm) coating of 10B. There have been two approaches used in the prior art for laying down such coatings. One technique utilized by many manufacturers has been a chemical adhesion technique. A suspension of small 10B particles was introduced into the detector tube and allowed to evaporate at high temperature until a liquid binder solidified and bound the 10B particles to the detector wall. This method had many drawbacks including (1) residual binder was always present at high enough level to reduce the escape efficiency of the charged particles and hence lower efficiency; (2) the binder, depending on its composition, could introduce outgassing components into the counting gas and result in aging of the detector; (3) it was extraordinarily difficult to produce a uniform coating that was accurate to tenths of a micron causing variability in detector response along the longitudinal axis of the detector; (4) it was extraordinarily difficult examine the coating inside the tube after being deposited because the only entry into the tube was a narrow gas file port; and (5) many detectors utilizing this method demonstrated detachment of the thin film as the detector was exposed to thermal cycling and physical shock and vibration. The chemical binding employed by the method suffers from inconsistencies in procedure and from the basic weakness of the chemical glue like bond.
In a second method that was develop by Dr. Lacy, a sputtering technique was used to deposit enriched boron carbide on the thin substrate. As discussed in U.S. Pat. No. 7,002,159, the initial boron-coated straw detectors were made using ribbons of 10B coated material and helically winding those coated ribbons with a second ribbon having no coating as an outer overlapping layer (i.e., one over the other) with application of a very thin fast setting adhesive layer onto a precision cylindrical mandrel, producing a bonded and rigid, cylindrical detector straw.
The 10B ribbons were formed through vapor deposition of boron carbide (B4C), vapor deposited on aluminum or Mylar foils using a plasma deposition process. In the described embodiment, deposition was accomplished by wrapping narrow 9.5 mm wide and 25 μm thick strips or ribbons of the respective materials around a cylindrical drum (16″ diameter×16″ long), which thereafter was rotated adjacent to a sputtering head. A tape running down the side of the drum kept the strips in place (but also produced a 1 cm dead space every 50″). Using this process, a uniform boron carbide coating was achieved on continuous strips of material with a length up to 50 meters. Prior to straw construction the coating quality could be easily evaluated by very simple tests such as application of mechanical and thermal stress to assure 100% bonding reliability.
In practice, the method of producing the '159 patent coated ribbon was limited. This batch process required substantial time in preparation and pump down which limited the production capacity. Further, because the size of the production run was limited to the material fitting on the drum, therefore each production run produced a small quantity (approximately 0.8 m2 per day at a cost of over $3700 per/m2.
As can be seen, as the need for neutron detection systems expands, and boron-coated straw detector systems replace 3He detectors in many applications, there will be an increasing need for a method of manufacturing greater number of boron-coated straws for these detectors. Since the prior art process of manufacturing straws was limited by the amount of foil that can be coated, there exists a specific need for a better process that can produce the quality of boron-coating on the foil in increasing quantity.